BEIJING — A 3.5-magnitude earthquake was detected in northern North Korea on Saturday afternoon, near the nation’s known nuclear test site, according to the U.S. Geological Survey.

“This event occurred in the area of the previous North Korean Nuclear tests,” the USGS said on its website. “We cannot conclusively confirm at this time the nature (natural or human-made) of the event.”

The earthquake-monitoring agency in neighboring China said it suspected that the North Korean quake, which occurred at 3:59 p.m. local time, was caused by an explosion, though the magnitude was significantly lower than a previous nuclear test earlier this month. The USGS estimated the depth of Saturday’s quake to be five kilometers (three miles).

But South Korea’s meteorological agency said the incident appeared to be a natural quake.

An official from the agency, speaking to the Associated Press on the condition of anonymity citing office rules, said an analysis of seismic waves and the lack of sound waves clearly showed that the quake wasn’t caused by an artificial explosion.

North Korea’s last nuclear test — its sixth in total — was detected as a 6.3-magnitude earthquake Sept. 3 and was followed by a 4.1-magnitude quake that experts said could have been a tunnel collapsing after the nuclear explosion.

In a tweet, nuclear proliferation watchdog CTBTO said it was examining “unusual seismic activity of a much smaller magnitude” in North Korea. Executive Secretary Sina Zerbo tweeted that it had occurred about 50 kilometers (30 miles) from the site of prior tests.